Spray control systems are used for applying paints and other coating compositions to a wide variety of plastic, metal, and optical components. In many of these applications, it is beneficial if not important to achieve a uniform thickness of coating material. Consequently, spray control systems typically utilize closed loop control to regulate the flow of coating material to the spray gun or other applicator. These closed loop control schemes use the error between the measured flow and a setpoint to provide negative feedback that drives the flow toward the setpoint. PID (proportional plus integral plus derivative) control is common among these control schemes.
While closed loop control provides accurate regulation of the flow to the setpoint, there are some inherent shortcomings in its use in spray control systems. First, since the typical closed loop control schemes rely on an error between the setpoint and measured flow, the controller must wait for the flow to begin and be measured so that it can determine the error needed for adjusting the flow control valve. This causes the flow to ramp up to its setpoint each time the flow is begun. Secondly, in control systems that utilize PID loops, the initial flow as a function of time can be dependent on the flow rate selected, with the initial flow ramping up exponentially for some flow rates and oscillating or ringing for others. Consequently, these control systems are sometimes tuned for a particular flow rate and do not react as desired when other flow rates are selected by the operator.
Mass flow meters are sometimes used in spray control systems. These meters typically provide a pulse train having a repetition rate that varies with the flow of material past the meter. Conventional spray control systems determine the flow rate by counting how many pulses are received from the meter over the course of a period of time. While accurate, this approach is somewhat slow and can cause a delay in both the feedback loop and the update time for the control system's flow rate display. Often, these displays are digital readouts that use a number of conventional seven-segment LEDs to provide a digital display of the flow rate. The display is usually controlled with a single conventional driver that receives a train of clock pulses that are counted by the driver and used in determining which of the segments of each LED should be illuminated. Then, to display a particular number, the driver is simply provided with that number of clock pulses. However, when this conventional display scheme is used for multi-digit numbers, the time required to update the display with a new number can become significant due to the number of clock pulses that must be generated and counted. For example, a four digit number requires a possible maximum of nearly 10,000 pulses.
Accordingly, it is a general object of this invention to provide a spray control system which improves the speed of the control system, both in terms of the time it takes to achieve and maintain a selected flow setpoint and the time it takes to obtain a new flow reading from the flow meter and update the display.